Practical Domestic
Room Acoustics.
© 2006 Adam P. Salisbury
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Introduction.
Few have the luxury of a purpose-designed, dedicated room in which to listen
their hifi system. Inevitably, therefore, most systems will be used in a room that is
of shared function, as well as shared occupancy. Yet shared function, even
shared occupancy, need not deter acoustical treatment so vital to the qualitive
reproduction of music. One simply needs to look at low to minimal impact OEM
solutions and the intelligent use of viable domestic alternatives.
But why?. It is not an unreasonable, or uncommon question. Indeed, without
acoustical treatment, many systems will still be capable of high quality
reproduction. The answers lies; as it so often does, in the detail. More specifically
the micro detail, or that which defines qualitive hifi reproduction.
For those who prefer a simile, think of those Magic Eye™ pictures. To the
untrained eye the flat (coded) image has visual merit, in much the same way as
the majority of the general public find musical satisfaction from a midi or mini
system. Hifi equates to the viewer who looks below the surface and begins to see
the outline of a 3-dimensional image. The higher we move up the hifi ladder, the
greater the 3-dimensional detail. However, as much effort as we expend
increasing 3-dimensional detail, the image doesn’t lift from the background, and
the rest of the picture remains out of focus. Room acoustics adds that focus.
In audible terms the soundstage ceases to be an arbitrary width and depth,
rather a facsimile of the venue, complete with ambience cues. Height ceases to
be (solely) determined by drive unit placement, and has the ability to extend both
above and below the loudspeakers. Width and depth are not a single dimension,
rather they are layered. Instruments are individual as part of the whole, rather
than being lumped together. You can sense the air between performers. Perhaps,
though, the vocal is the most indicative quality of all. Yes it's central, yes you can
hear instruments and performers either side of, and behind it, but it is the way
that it lifts from the soundstage and becomes truly three-dimensional.
In Simple Terms.
The subject of acoustics can be physically, theoretically and mathematically
complex. To do real justice to the subject requires in-depth analysis of all of
those factors, wrapped in a dictionary-worth of terminology and abbreviations.
That is not the intention of this article. This article is designed to be widely
accessible and practical. That is not to say that all terminology can be, or indeed
should be discarded. Yet, what does remain will mirror the article as a whole, in
being accessible and practical.
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Acoustical Frequency Range.
The audible frequency range of 20Hz – 20kHz is most commonly split into bass,
mid-range and treble – to basically partition low, middle, and high frequencies.
The same frequency range undergoes a similar split in acoustical terms. This
defines the interaction of different frequency ranges with the listening room, as
well as their treatment.
For the sake of simplicity this article will use an arbitrary split at a frequency of
300Hz. Below 300Hz can be considered modal, and above 300Hz can be
considered specular.
Modal:
This is the range of frequencies that the listening room primarily supports as
standing waves (although in real terms there is an upper and lower limit). A
standing wave being a frequency whose wavelength allows it to fold back on
itself (via reflection), such that it’s peaks and troughs coincide – i.e. ‘stand’. This
begins to occur when the wavelength equals twice the longest room dimension,
or calculable by 1130/2L (1130 is the speed of sound in ft/s, and 2L is 2x the
longest room dimension in feet). Thus a 20ft long room: 1130/(2x20) = 28Hz.
Bear in mind that you will have integer multiples of that frequency, as well as
similar patterns from the other two axial (wall to wall, floor to ceiling) dimensions.
The modal range, then, is a function of room dimensions and reflections. This
range can be subject to extreme variations in frequency response, which are
difficult to treat given the large impacting wavelengths. This is especially true
when two or more room dimensions match, or when they are divisible multiples
(i.e. 16ft width and an 8ft ceiling). More modest anomalies can be successfully
treated, but even modest treatments often have maximal visual impact.
Specular:
As in capable of reflecting light. Frequencies in this range acting in much the
same way as light does. On impacting a surface, waveforms are reflected from it.
The angle (of incidence) at which a waveform impacts a surface will equal that at
which it reflects. This region is highly susceptible to room treatment, and smaller
impacting wavelengths means that treatment can have minimal visual impact.
Absorption.
Sound, incident upon a surface, will be reflected from it. The level of reflection (in
relative terms) is determined by the frequency of the impacting sound, and the
level of absorption of the impacting surface. By varying the absorbency of the
impacting surface, so we gain control of reflections from it.
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The level of absorbency of a material is known as its Absorption Coefficient. The
level of absorbency, itself, is relative to the frequency of the impacting
wavelength – some materials are especially good at absorbing lower frequencies,
yet have poor absorption at higher frequencies. And vice versa. Material testing,
therefore, must take place across the frequency range. Naturally, this is
impractical, hence the standardised use of six (octave spaced) testing
frequencies.
The six testing frequencies chart the performance of a material over a wide
frequency range, allowing the end user to select a material that is effective over a
wide bandwidth, or one that has good absorption at a more specific frequency
range. To define this performance absorption tests at each of the six frequencies
derive a figure between 0 and 1. 0 equating to no absorption, and 1 equating to
complete absorption. Each figure also translates into a percentage – a material
having an absorption coefficient of 0.40 absorbs 40% of the waveform incident
upon it, and reflects 60%.
Tables derived from material measurement exist in abundance, and not just for
acoustical products. As materials deviate, though, from a specific product to a
domestic equivalent (i.e. from a known make of mineral wool insulation to
curtains) so the accuracy of the absorption coefficients suffer. However, with
averaging it is possible to draw up a list of absorption coefficients that are
applicable to basic OEM materials, as well as their domestic equivalents:
Material
Wood laminate flooring
on concrete
Standard carpet on
concrete
Heavy carpet plus
underlay on concrete
Plaster on brick walling
12mm plasterboard on
studwork walling
Window glass
Lightweight cotton
curtains
Heavyweight cotton
curtains
Heavyweight cotton
curtains 4” from wall
Heavyweight velour
curtains
25mm acoustic foam
50mm acoustic foam
100mm acoustic foam
125Hz
250Hz
500Hz
1kHz
2kHz
4kHz
0.04
0.04
0.07
0.06
0.06
0.05
0.05
0.12
0.40
0.70
0.60
0.40
0.15
0.25
0.50
0.60
0.70
0.80
0.01
0.02
0.02
0.03
0.04
0.05
0.29
0.10
0.06
0.05
0.04
0.03
0.35
0.25
0.18
0.12
0.07
0.04
0.04
0.05
0.11
0.18
0.30
0.44
0.05
0.12
0.35
0.45
0.44
0.40
0.09
0.33
0.45
0.52
0.50
0.44
0.14
0.35
0.55
0.72
0.70
0.65
0.08
0.10
0.20
0.13
0.25
0.70
0.30
0.60
0.99
0.68
0.90
0.99
0.94
0.95
0.99
0.99
0.99
0.99
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50mm mineral wool
100mm mineral wool
0.17
0.32
0.62
0.98
0.90
0.99
0.90
0.90
0.88
0.88
0.82
0.82
It is important to bear in mind that the level (percentage) of absorption, whilst
relative to the material type, is also relative to the unit area of the material used.
1 sqft of material that has an absorption coefficient of 0.62 at 1kHz does not
reduce the room’s overall response to 1kHz by 62%, rather it reduces those 1kHz
waveforms that impact it. Those waveforms that do not impact it are not affected.
The room’s overall response to 1kHz will be reduced, but only by a very small
amount. By increasing the unit area of absorption, so overall response will
reduce.
The inquisitive reader should look to the work of Wallace Sabine, and the unit of
absorption, the Sabin. Suffice to say here that determination of a room’s
absorption tends to be relative to the design of professional acoustical spaces.
Experience suggests that domestically there is (usually) just below sufficient inroom absorption, leaving latitude for modest absorbent treatment, without
recourse to Sabin calculation.
Diffusion.
Diffusion, in an acoustical sense, relates to the scattering of sound that is
incident upon a diffuse surface. An absorbent panel reduces the level of the
reflected waveform relative to the frequency of that waveform, and the absorption
coefficient of the panel at that frequency. The angle of reflection (albeit at a
reduced level) is essentially unaffected.
On impacting a diffusor, the incident waveform is scattered in a pattern relative to
the type and design of the diffusor. For the sake of simplicity we will assume the
scatter pattern to be equal about 180 degrees. Thus, an element of the reflected
waveform will have a path similar to that from an absorber, albeit at a much
reduced level. The difference being that similar reflections will also occur
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throughout 180 degrees. In other words, the level of the impacting wavelength
has not been reduced by being absorbed*, rather it has been reduced by being
broken up (essentially into multiple reflections) and scattered about 180 degrees.
In order to function properly, a diffusor needs to be made of a rigid material.
Rigidity usually means density, and density often translates into weight. Thus,
whilst concrete may be ideal, it is hardly practical. Wood is a good alternative, but
even small diffusors soon become very heavy, and difficult to install. In reality
there is a compromise between rigidity, weight, and thus performance. *It is
inevitable, therefore, that most diffusors also have a modest element of
absorption.
Practically speaking any rigid device or uneven surface in the path of an
impacting waveform will act as a diffusor. Naturally, there are functional provisos.
The first being the size of the diffusor relative to the size of the impacting
wavelength. Wavelength being simply calculated by: 1130/f (1130 is the speed of
sound in ft/s, and f is the target frequency). Thus: 1130/1000 = 1.13ft. So, a 1ft
square panel will be ‘seen’ by all frequencies down to about 1kHz (in reality there
will be some function up to an octave (f/2 or 500Hz) below that figure).
Secondary provisos come from the type and design of the diffusor, but mostly
deal with the uniformity and direction of spread from the diffusor. These are
mathematically derived (and complex) designs that determine the dimensions of
the individual elements that make up an OEM diffusor panel. To go into any
depth here would detract from the purpose of this article. However, the inquisitive
reader can seek further information on Quadratic Residue and Primary Root
mathematics, and consequent OEM products such as the Skyline and Omni from
RPG.
It is important to bear in mind that negating the secondary provisos does not
remove the use of a rigid device or uneven surface as a means of diffusion – it
merely means that it’s diffusion characteristics are harder to predict.
Reverberation.
Reverberation is the sustaining of sound as a result of reflections within the
listening room. Reverberation spans the frequency range, so that you can have
low frequency reverberation as well as high frequency reverberation. Following
the cessation of the exciting source (music from a system) reverberation will
diminish over time. The time taken for reverberation to fall by 60dB is known as
the Reverberation Time (or RT60).
RT60 is determined by the size of the listening room, and the level of absorption
therein. As the size of the domestic listening room is a constant, so RT60 is
controlled by the level of absorption. Bearing in mind that all absorption is taken
into account, not just that added as an acoustical treatment. This means walls,
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floors, the ceiling, and all furnishings. Even People. Varying the number, and/or
material of any of which, will affect RT60.
Naturally, this is calculable down to the nth degree. Fortunately, we can simplify
matters (with reasonable accuracy), using averaging. A normal multi-function
room (average British sized, fully carpeted, 3-piece suite, etc) used as a listening
room, may have an RT60 of 0.6 (seconds). A similar room, but sparsely
furnished, using a wood-laminate floor may increase to 0.8 or higher. A modestly
treated near ideal room would be 0.4 – although this is subjective according to
the type of music preferred.
The consequences of excess reverberation are a loss of vocal legibility, which
occurs rapidly for minor increases in RT60. Further, space within the soundstage
becomes echoic rather than naturally ambient, with a consequential loss of
image placement. On the flip side, a very low RT60 results in a dead response.
Vocal legibility is high, but there is no life to that vocal. Stereo separation is acute,
but without the low level detail that defines soundstage and image placement,
very two-dimensional. Indeed, not unlike headphone reproduction.
How, then, can RT60 be tested, retaining the simplicity of focus of this article?.
There are two ways. First, via the Internet, where a number of websites host
downloadable .wav files recorded to illustrate the difference (usually of speech
legibility) between various RT60 times. A second, and oft-used test, is the hand
clap. Clapping ones hands in the listening room will give an idea of mid to high
frequency reverberation. This method will not generate an actual RT60 time, but
it can indicate the reverberant tail (flutter echo) of an overly reflective space.
Fortunately, experience suggests that major problems in terms of domestic RT60
are rare. The modern trend towards sparse furnishing and laminate flooring is
detrimental, but readily treatable. Indeed, the application of ‘first reflections’
treatment usually accounts for RT60.
System Set-up.
System set-up primarily concerns loudspeaker and listening position placement.
One can not be considered without the other, and together they have a marked
effect on system response. Minor anomalies in the modal range can be negated
with careful loudspeaker/listening position placement, and specular (particularly
first reflections) tempered. In a domestic situation where minimal visual
alterations can be made (in terms of room treatment), then loudspeaker/listening
position placement become critical. Compromise without recourse to room
treatment and system response will suffer.
In terms of actual placement, that is left to the end user. Many procedures exist
for such placement, and most experienced users have adopted a procedure that
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they are familiar with. Those that have not should consider that from Audio
Physic.
http://www.immediasound.com/Speakersetup.html
Room shape and other issues.
Room shape, windows, doors, stairwells, even the presence of a television all
have an effect on system response. Their effect is often detrimental, but it need
not be markedly so. Indeed, many problems are avoided by careful loudspeaker
and/or listening position placement, alongside modest room treatment. Perhaps
the main requirement is that of balance. Room layout either side of each
loudspeaker need not be identical (ideally it would be), but major variations
should be avoided.
Chimney brests and alcoves:
A common feature of (particularly) older British properties. Extending perhaps 8”
– 12” into the room, a chimney brest is a seemingly ideal (domestic) solution to
otherwise intrusive loudspeaker placement. Yet, whilst domestically ideal, it is
demonstrably less so when its effect is considered.
Tucked into the sides of a chimney brest the loudspeaker baffles (the front panel
of the loudspeakers on which the individual drive units are mounted) are level
with (or close enough to be considered level with) the front wall of the brest. As
all but the very highest frequencies effectively disperse hemispherically from the
baffle (initially), so the front wall of the chimney brest acts as an extension of
each baffle – radiating sound towards the listener in much the same time frame
as that from the loudspeakers themselves.
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This has two dominant effects: firstly you lose some degree of channel
separation, in favour of an artificially strong central focus. This narrows the
soundstage and negates imaging (particularly depth) in favour of a twodimensionality, not unlike headphone presentation. Secondly, as high
frequencies are less affected (due to their narrow dispersion), loudspeaker
location becomes acute, but their perceived balance will likely be lean in the mid
to low-mid bass.
Placement either side of a chimney brest has another primary effect – bass
loading. At the very least such placement will load the loudspeakers from two
surfaces, perhaps three if the loudspeaker design is floorstanding or has a
separate low-slung bass module. This will cause near maximal modal resonance,
which even in a room of modest response will almost certainly result in a lumpy
low bass response, and perhaps a monaurally-perceived bass boom.
The remedy is simple – bring the loudspeakers forward so that as an absolute
minimum their backs are flush with the front wall of the chimney brest. This will
not wholly negate the effects mentioned above, indeed it may breed additional
diffraction effects from the edges of the brest, but overall response will improve.
Ideally the backs of the loudspeakers should be at least 6” – 12” from the front
wall of the chimney brest. 12” or more and the effects of the brest can be
negated, or at least considered of secondary importance.
Alcoves and other indentations in the room follow similar principles – they should
not be considered as convenient places to ‘hide’ loudspeakers.
Doors:
Doors themselves are rarely problematic in terms of domestic acoustics, and it is
not necessary to go to the extremes that professional studios do with their
hermetically sealed double units. That said, their consideration in one or two
areas is of value.
Firstly, and rather simply, is the consideration whether the door be left open or
closed. Clearly, an open door is a ready means of sound transmission to other
parts of the house, which is often unacceptable. However, an open door will also
affect the bass response of the room, particularly at very low (including sub
modal) frequencies.
Wavelengths that exceed the maximum room dimension cease to be supported
by the room in terms of modal resonance. Instead they are considered in terms
of pressure, and constitute what is often referred to as ‘room gain’. Very simply,
room gain is the phenomena responsible for a potential 12dB/octave worth of
gain at very low frequencies, extending potential system response.
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In reality, such a high level of gain is unlikely, and in many respects any such
gain is dependant on the design and construction of the room. Ideally, in terms of
room gain effect, the target room should act as a sealed chamber – any deviation
from that and the potential effect will be reduced. Naturally, few rooms are air
tight, even with their door(s) closed, although closing the door does increase the
potential for room gain.
A secondary consideration is that of mechanical interference. Doors and their
frames are invariably of wood construction, using metallic hinges and closings.
Subject to even modest low bass response a door will flex and vibrate. The wood
to wood and wood to metal interactions, when vibrated, often cause audible
mechanical interference. A thin strip of draught-excluding foam not only cures the
problem, but assists in sealing the door against unwanted transmission.
Adjacent rooms and stairwells:
This situation is really an extension of that above – albeit doors that are left open.
Therefore, it is inevitable that the primary effect of an adjacent room or stairwell
is on low bass response. Although that is not the only consideration.
An adjacent space, whether it is accessed from a doorway, archway or stairwell,
is essentially an extension of the listening room. As such the air mass within that
space will be modally excited in the same way as that in the listening room. More
often than not the effect of this is a reduction in apparent bass level in the
listening room. The ear being markedly less sensitive to bass frequencies
perceives this as a lean response, affecting the reproduction of low frequency
instruments, but also the lower vocal range. A situation exacerbated by the lack
(or severe reduction) of sub modal room gain.
This shift in bass balance has a consequential emphasis on higher frequencies,
often giving the (wrong) impression of a bright or treble-heavy system. That said,
consideration must be paid to the loudspeakers themselves, and their ability at
reproducing low frequencies. Those designed to operate clear of room
boundaries will have natural low frequency extension, and suffer far less than
those designed to utilise boundary loading and room gain.
Whilst important, the above situation is not automatically detrimental. The proviso
being that loudspeaker placement is symmetrical in relation to the adjacent
space. In other words, the left-hand loudspeaker ‘normally’ placed in the listening
room will have a markedly different response to the right-hand loudspeaker
placed close to an adjacent space, open doorway or stairwell.
Remedy here is mostly achieved through symmetry of loudspeaker placement
about the adjacent space or opening. Where this isn’t possible, or in order to
simply control the effect of the adjacent space or opening, one needs to consider
some form of temporary absorbent treatment. It is less easy to treat a stairwell in
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this manner, but a doorway or archway is easily covered with a curtain
(preferably heavily pleated heavyweight cotton material). When this situation
refers to asymmetrically located loudspeakers, then the wall adjacent to the
‘normally’ placed loudspeaker should also be treated in a similar manner. With
the system not in use the curtain(s) can be drawn back, and hence have minimal
visual impact on the room.
An alternative to the curtain, that finds some use because of it’s decorative
nature, is the folding three-panel screen. These are often used in a non-acoustic
sense as a temporary room divider, so their use acoustically is a natural
progression.
Bay windows:
A bay window is rarely benign, but its effect is relative to loudspeaker/listening
position placement. Worst case scenario, experientially derived, is when
loudspeakers are positioned either side of the bay – the bay itself is often seen
as a good (domestic) repository for an equipment rack.
Such loudspeaker placement has two primary problems. Firstly, the concave bay
acts in much the same way as any concave surface (a mirror or a satellite dish,
for example), in that it ‘gathers’ (in this case sound) and reflects it to a focal point.
If the listening position is facing loudspeakers placed either side of a bay window,
it is inevitable that part of the output from the loudspeakers will be re-focussed by
the bay, towards the listener. As the highest frequencies have limited dispersion,
so much of this focussed information will be mid-range and below.
The effect has much to do with the size and design of the bay, and the placement
of the loudspeakers/listening position. However, it is likely that channel
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separation will be compromised in favour of an artificially enhanced central focus.
Unlike the response from loudspeakers placed level with a chimney brest (above),
it is likely that this central focus, whilst predominant, is perceived as hollow, or
slightly echoic (particularly in terms of vocal response). This effect can be
mistaken for ambience or image depth. Soundstage width will also be
compromised.
Secondly, a bay window can act as a ‘soak’ to low frequencies, affecting low to
mid bass performance. This is very much determined by the design of the bay,
particularly the size and type of glass. As larger and/or single panes readily
vibrate at low frequencies – and vibrational energy is turned into heat – so glass
can be considered a reasonably effective low frequency absorbent (see the
absorption coefficient table above). Naturally, this affects all glass – alongside
the potential for mechanical vibrational interference. Where bay windows differ is
in their concave shape which gathers and re-focuses sound, but also their
increased glass surface area.
Following the worst case scenario would be the bay window to the side of the
loudspeakers/listening position. Whilst this would effectively negate the concave
focussing of sound, it is possible that the bay would still act as a soak to lower
frequencies. As the loudspeakers would be asymmetrically located, so that
nearest the bay may well suffer from a leaning of its low frequency response.
The remedy, then, must be to have the bay to the rear of the listening position,
the loudspeakers outputting towards it. There would still be some focussing as
sound entered the bay, but initial loudspeaker output would reach the ears intact,
and thus the ear/brain interface will prioritise and localise from that. To assist the
situation, or as a simple remedy for side-wall and either side of the bay
positioning, then one should consider the use of heavyweight cotton curtains
drawn across or around the bay.
The trend towards lighter material curtains and/or blinds will prove far less
effective than heavy cotton. Even using heavy cotton they will need to be fully
drawn to have full effect – something off-putting to most people during daytime
listening sessions. A less effective, but visually attractive alternative is a large
potted plant (or collection thereof). Studies (below) have highlighted the realworld absorption of the larger palm-type houseplant, and anecdotal evidence
exists that shows modest levels of diffusion too.
Other:
This will be an ongoing collection of personal and received observations, often
those that require nothing more than consideration and careful placement/use.
The primary concern is that initial loudspeaker output should reach the listener
intact. This means that the area between the loudspeakers and the listening
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position must be kept clear of (particularly) coffee tables and other hard reflective
surfaces. This initial output allows the ear/brain interface to localise output, to
create an accurate central focus, and for them to combine in creating an accurate
soundstage and image.
Of equal import is the area between the loudspeakers themselves. It is
increasingly common – especially in combined audio-visual systems – to have a
large-screen television in-between the loudspeakers. Whilst this is ideal for
centralising speech and for effects steering in movie playback, it is detrimental to
audio quality in much the same way as a chimney brest. Naturally, this applies
more to qualitive hifi reproduction than it does to the aurally inferior movie
soundtrack reproduction. Beyond relocating the television, remedy is much the
same as that for the chimney brest.
The effect of other items placed between the loudspeakers really depends on the
items themselves – the most common being the equipment rack. Although, whilst
there may be some diffraction effects (depending on the proximity of the
loudspeakers), it is unlikely to be sonically detrimental. Where possible, though,
the loudspeakers should be forward of the rack to minimise any potential
interaction (and this applies to other items thus placed).
Perhaps curiously, the most overlooked ‘issues’ are often the simplest. Groups of
ornaments (particularly china or glass) that touch each other will vibrate, and
have their own (low level) sonic signature. Glass shelves are another oversight,
otherwise firmly fixed, they are apt to vibrate on the metal ‘L’ brackets that are
often used to hold them in place. Light fittings (particularly glass shaded), given
their usual boundary location are also susceptible to low frequency vibration, and
loose fittings will cause audible mechanical interference. Gas and solid fuel fires
both have components that can and will vibrate. And large CRT televisions, with
their vast plastic panels (particularly the removable rear panel), can produce an
extremely invasive vibrationally-derived interference.
Singly these ‘issues’ may not be overly detrimental. Indeed, unless in isolation,
they may not be wholly audible. That said, they will always affect reproduction,
even if only subtly. Fortunately, then, their remedy rarely needs more than a little
time and thought. Well, that and perhaps a pack of the ubiquitous Blu Tac™.
Room treatment.
Modal Treatment.
Experienced listeners will know when their system is at fault in terms of low
frequency reproduction, without recourse to acoustical measurement. Lower
frequency wind instruments and drums, and particularly string instruments, lose
clarity. Notes cease to be singular as part of a whole, blurring into one, a sort of
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ill-defined ‘warmth’ in place of detail. Or, worse still, a monaurally perceived
boom, interspersed by a few overly dominant notes (frequencies).
What few may realise is that problems at lower frequencies affect those
frequencies above. Not just those instruments (or vocals) that span a large part
of the frequency range, but those that don’t too. Accurate musical reproduction is
all about balance, relative to that from the recorded performance. Any shift in that
balance is at least a shift in accuracy, but also a potential shift in perception.
Unfortunately, low frequency (modal) treatment often has maximal visual impact.
In a shared function domestic environment this is rarely acceptable, leaving
loudspeaker/listening position placement as the only potential remedy. Quite
often, this alone will prove extremely effective at, if not curing, then ameliorating
modal anomalies. Without recourse to modal room treatment, freedom of
loudspeaker/listening position placement becomes critical. Compromise here and
you will compromise system performance.
For those with a little more latitude there are modal treatments that whilst having
visual impact, have only moderate dimensional impact, or at least a reasonable
aesthetic. The primary example of which is the membrane (or resonant panel)
bass trap, which work by turning vibrational energy into heat. These active bass
traps consist of a wall-mounted membrane (or panel) – contained within a
framework – that have a resonant bandwidth determined by the membrane
(panel) material, and the depth of the cavity behind the membrane (panel). The
bandwidth will be sufficient to cover most of the modal range, and with the right
combination of material and cavity depth (at least 4” to 6”), have good absorbent
action below 100Hz. Modex (including Modex Corner), from RPG, is a good OEM
example of this type of treatment.
http://www.rpg-europe.co.uk/Products/modex/modex.htm
Shaped foam is a functionally simpler alternative. Available in 3ft or 4ft lengths,
often with a profiled face (to increase apparent depth), such products work via
the simple (passive) absorbent action of the foam. Depth of product needed
tends to restrict their design to corner placement, which itself is beneficial in that
corners are the site of maximal modal concentration, and thus effect. Procorner
from RPG, is a good OEM example of this type of treatment.
http://www.rpg-europe.co.uk/Products/procorner/procorner.htm
In terms of placement, much depends on the design, and OEM products will
come with guidelines from the manufacturer. Utilising DIY alternatives and/or for
generalised treatment: consider room boundaries, and particularly the number of
intersecting boundaries – the greater the number, the greater the modal support.
Hence the design for, and the use of, corner placement.
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There are provisos. At boundaries air motion is at its minimum, but pressure is at
its maximum. This means that passive treatment (such as foam) has reduced
absorbent affect, hence the design for, and use of corner placement – as the bulk
of the foam is spaced from the intersecting boundaries by the corner, and thus
provides useful absorbent action. Conversely, active membrane (panel) traps
remain active at boundaries, converting pressure waves into heat via the
resonant action of the membrane (panel). Further, as the vibration of the
membrane (panel) also produces air motion, the effect of such traps can be
improved by the addition of an absorbent in the cavity.
Unless dictated by design, active traps tend to have more latitude in terms of
absolute placement. This means that they can be flat wall mounted for minimal
visual and dimensional impact, but they will have added effect if used to span
corners or wall-ceiling intersections.
Naturally, the equalising or levelling effect (in terms of modal frequency response)
of these wide bandwidth products is relative (a) to the amount of
equalisation/levelling required, and thus (b) the number of products used.
Experience suggests that many rooms (of modest modal response) react well to
just two such products – used in the corners to the rear of the loudspeakers
(which is fortunate in terms of potential domestic acceptability). Further gains
may be realised from treating the corners to rear of the listening position, and
then via additional placement at the side wall-ceiling intersections.
Electronic modal treatment:
Despite their increasing popularity – especially in combined audio-visual systems
– such units go against the premise of this article, and relative to many
performance factors, can not be recommended in terms of qualitive hifi
reproduction.
There is one exception, and that is with rooms that have a very poor modal
response (same or divisible room dimensions – a square room for example). Few
such rooms will be wholly treatable by passive acoustic means, and those that
are will require the use of a large number of both wide and narrow bandwidth
products – which will naturally have maximal visual and dimensional impact.
In such cases the inquisitive reader should investigate the range of inexpensive
products from Behringer, such as the Ultracurve Pro:
http://www.behringer.com/DEQ2496/
There are numerous internet-based communities that review the use of such
products, and advise on their application. If simple guidelines are required – in
terms of limiting their potential detrimental effect – then consider manual use
15
over measurement microphone derived auto set-up, and do not use boost (gain),
only cut (attenuation) to deal with peaks in (low frequency) response.
Specular Treatment.
The initial (direct) signal from the loudspeakers is critical. It allows the ear/brain
interface to localise output, to create an accurate central focus, and for them to
combine in creating an accurate soundstage and image – for more detail the
inquisitive reader should refer to the work of Helmut Haas and the
Haas/Precedence Effect. Suffice to say here that the ear/brain interface is
remarkably adept at doing this, even having a degree of latitude in which indirect
(reflected/diffracted) signals (below a certain time frame and/or level) will be
merged with the direct signal, with no loss of accuracy of effect.
Indirect signals that exceed this latitude in terms of time of level may be heard as
distinct, yet additional point sources, or simply as echoic. In either case they will
affect central focus and soundstage and image perception. It goes without saying
that micro detail – that which defines qualitive hifi reproduction – will be
irretrievably lost. It doesn’t so much make sense to control interfering reflections,
rather it defines potential system performance.
First reflections:
If the output signal from the loudspeakers is considered as the direct sound (that
which reaches the ear first), then first reflections are those that impact a single
surface on the way to the ear. There are four reflection points that are of primary
interest: the floor, the ceiling, and the two side walls. It is these that the reader
should concentrate on, identify, and treat.
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Identification can only take place when the loudspeakers and listening position
placement have been decided upon. For the process itself you will need a mirror,
and; to make life easier, an assistant. While seated in the listening position have
the assistant move the mirror along an imaginary line that runs on the floor
between the left-hand loudspeaker and listening position. When the tweeter/mid
of the left-hand loudspeaker can be seen in the mirror, mark that point on the
floor. Repeat this action for the ceiling. Similarly, for the left-hand wall (with the
mirror at around tweeter/mid height) – note the position on the wall when the
tweeter/mid of the left-hand loudspeaker can be seen in the mirror. Repeat for
the right-hand loudspeaker.
This process should create six marks, two each on the floor and ceiling, and one
on each side wall. More often than not the two marks on the floor and on the
ceiling are in reasonable proximity, and sufficiently close to be treated as one
point. These, then, are the four primary reflection points. Time for treatment.
Treatment itself refers either to absorption or diffusion. They each have their
individual merits – absorption through reducing reflected energy, and controlling
reverberation, diffusion through redirecting or spreading energy, increasing
perceived space and ambience. And, in combination, they will have a marginally
different overall effect. There are few provisos (other than practicality) that dictate
the use of one over the other, save that diffusion needs ‘space’ in order to
function effectively. Therefore, diffusion should not really be used where the
impacting surface is less than 2m from the listening position.
Practicality determines that the floor be treated with an absorbent, and all but the
thinnest of carpets is usually sufficient in this regard – there is no real need for an
OEM or specific acoustic treatment. Those that have jumped on the latest
domestic-fashion bandwagon that is wood-panel flooring, need to use a thick-pile
17
rug that not only spans the identified reflection points, but that really covers much
of the area between the loudspeakers and the listening position. Such a covering
is rarely domestically unacceptable, although it is wise to countenance opinion of
one’s partner in this purchase.
Less readily tolerated are changes and/or additions to the walls and the ceiling.
With the ceiling being the more difficult as there are few domestic alternatives.
Various forms of lighting have been considered – fluorescent strip, low-voltage
track, recessed spots – and all have been found wanting. It would be possible to
use a suspended tapestry or decorative material as a form of absorption, but that
very much depends on the end user.
It is fortunate that the small wavelength size of impacting specular frequencies,
requires only modest depth of treatment. This means that 25mm of acoustical
foam will provide acceptable treatment. For those with more latitude 50mm will
be better, and it should be borne in mind that diffusion in this situation is an
excellent solution. One such example is the Low Profile (LP) Skyline diffusor from
RPG. These lightweight 2’ x 2’ (600mm x 600mm) panels (which can be supplied
in myriad colours) extend just 4” – when adhesive-mounted to the ceiling, yet
provide fully controlled dispersion over the target bandwidth. Two such panels
will provide 4’ x 2’ coverage, which is sufficient in most circumstances, and of
limited (yet architecturally aesthetic) visual impact.
http://www.rpg-europe.co.uk/Products/skyline/skyline.htm
The walls follow similar rules to the ceiling, although they are more readily
treated with domestic equivalents. Wall hangings and the ubiquitous bookcase
are both oft-quoted domestic forms of absorption and diffusion. If the wall
hangings are of some thickness (even bolstered by, say, 25mm of acoustic foam),
and preferably spaced from the wall by a couple of inches, and that the bookcase
be as randomly stacked as possible. Then both become increasing acoustically
acceptable too. It is difficult; perhaps impossible, to predict their acoustical
performance, but the experienced listener should be able to determine their
relative worth, in terms of system performance.
One point to note, and one that may have a determining factor on the
product/domestic alternative used, is that of symmetry. Floor/ceiling symmetry is
not necessary as we do not have discrete up and down channels, but adjacent
wall symmetry is, as stereo resolves around discrete left and right channels, and
particularly their balance. Thus, it is not advisable to have diffusion on one wall
and absorption on the other – they should both be absorbent, or they should both
be diffuse.
Experience suggests that absorption is the better choice for side wall use. This
has the consequential benefits that it is easier to achieve (OEM and
domestically), but also that free-standing panels (for use in front of windows and
18
other obstructions) are available (and readily replicated in a DIY sense). Primarily,
though, such treatment need only have minimal visual and dimensional impact.
As with the ceiling, 25mm of acoustical foam will provide acceptable treatment,
and for those with more latitude 50mm will be better. This can be turned into a
decorative panel, as typified by the Absorbor from RPG, which is supplied fabric
finished, in myriad colours.
http://www.rpg-europe.co.uk/Products/Absorbor/absorber.htm
Naturally, and despite the quality of finish of such products, beauty remains in the
eye of the beholder. And there is, admittedly, a ‘commercial’ appearance to many
OEM acoustical products – art they are not. Although, art they can become, for
an enterprising company: Decorative Art Services, do just that. Combining
original artwork with various OEM acoustical panels to create a range of bespoke
products that are both pleasing to they eye, and acoustically practical.
http://www.decorative-art-services.co.uk/
In terms of placement, much depends on the design, and OEM products will
come with guidelines from the manufacturer. Utilising DIY alternatives and/or for
generalised treatment: the floor and ceiling take their cues from the marked
mirror points. Usually they will be in reasonable proximity, and sufficiently close
to be treated as one point. Most OEM products are either 2’ x 2’ (600mm x
600mm), which is useful for spot treatment, or 4’ x 2’ (1200mm x 600mm), which
is sufficient to cover a side wall mirror point, or to span two ceiling mirror points.
Ceiling based treatment should horizontally span the marked mirror points, being
equally spaced about the centre of the absorbent or diffuse panel(s). Side wall
placement is a little different. Used vertically, the recommended 4’ x 2’ panel
should be about 18” from ground level, with it’s vertical centre line covering the
marked mirror point. This gives maximal coverage, often erring slightly toward
the upper part of the wall, where reflections are concentrated, free of impacting
furniture, etc.
The above information is relative to OEM treatment simply because much is of
standardised size, and thus it is easier to give reasonably precise placement
advice. Domestic equivalents are less readily predictable, although the premise
remains the same – the centre of the impacting surface (a wall hanging,
absorbent foam, bookshelf, etc) should be placed on the mirror point. Bear in
mind that a single shelf, which; naturally, is mounted horizontally, has less
impacting area when used on a side wall. Whilst it is important to note that this
alone is better than nothing at all, a second shelf (or more) would be an
advantage.
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IMPORTANT NOTE:
PLEASE BEAR IN MIND THAT THE INFORMATION ABOVE IS EXPERIENTIALLY DERIVED. IT ABSOLUTELY
SHOULD NOT DETER EXPERIMENTATION WITH ALTERNATIVE PRODUCTS, VARYING THE LEVELS OF
ABSORPTION, OR USING DIFFUSION IN PLACE OF ABSORPTION (AND VICE VERSA). THIS IS ESPECIALLY
TRUE AT THE SIDE WALL MIRROR POINTS.
Secondary reflections:
First reflections are of primary importance – and should be treated (literally) as
such. This, then, serves as an introduction to those reflection points of secondary
importance (relatively speaking), whose identification and treatment can be
considered after treating the first reflection points – a sort of acoustical upgrade
path.
Firstly, the front wall. Specifically that area behind, but in-between the
loudspeakers, which the listener faces. This may be a standard wall, or the front
of a chimney brest or fireplace. The latter can be quite useful, being a
combination of irregularly-shaped rigid surfaces. If there is a mantelpiece, a few
well placed books, or hard-material ornaments will provide a little diffusion that is
probably sufficient. Low impact OEM diffusion or absorption are valid
replacements.
Secondly, the front wall again, but this time that to the sides of the speakers. This
may be standard walling, or alcoves formed by a chimney brest between the
loudspeakers. Those areas of smaller dimension react well to ‘spot’ treatment, in
OEM terms a single 2’ x 2’ (600mm x 600mm) diffusor panel, centred and at a
height equivalent to the point between the tweeter/mid. As an alternative, and for
larger areas, shelving – perhaps for music media storage – is fine (even when
OEM is an option).
The rear wall, behind the listener, really depends on its distance from the
listening position. Those listening positions that are hard against the rear wall – in
smaller rooms, and/or when the (wider) horizontal dimension is used for
loudspeaker layout – should really consider the rear wall as a primary first
reflection point. It is important that an absorbent (panel or domestic equivalent)
be used here – diffusion simply doesn’t have the space to operate properly. As
the listening position moves further away, so the need to treat diminishes,
although the position is ideal for a domestic equivalent treatment such as a wall
hanging or set of shelves.
This leaves the side walls, which should already be treated at their first reflection
point. However, there is a second point, and one that really only emerges in
longer rooms. Using exactly the same approach as that for finding the first
reflection point – draw a mirror along the left-hand wall (at tweeter/mid height),
when you can see the right-hand (note: right-hand), mark that position. Repeat
for the right-hand wall (i.e. looking for the reflection of the left-hand loudspeaker).
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Treatment-wise, a light absorbent is sufficient – following similar guidelines for
installation as that for the first reflection point.
The role of plants.
Perhaps the most domestically acceptable addition to any room, plants also have
acoustical benefits. Research supported by the Rentokil company derived a table
of absorption coefficients for a variety of plant species, with anecdotal evidence
supporting likely diffraction and diffusion too. The research was primarily aimed
at commercial spaces, but it can be translated domestically. Hence, the
separation of this section of the article in order that it can be readily updated as
information becomes available.
Plant
Ficus Benjamina (Weeping
Fig)
Howea Forsteriana (Kentia
Palm)
Dracaena Marginata
(Madagascar Dragon Tree)
Spathiphyllum Wallisii
(Peace Lily)
Schefflera Arboricola
(Umbrella Tree)
Philodendron Scandens
(Sweetheart Vine)
125Hz
250Hz
500Hz
1kHz
2kHz
4kHz
0.06
0.06
0.10
0.19
0.22
0.57
0.21
0.11
0.09
0.22
0.11
0.08
0.13
0.03
0.16
0.08
0.14
0.47
0.09
0.07
0.08
0.13
0.22
0.44
-
0.13
0.06
0.22
0.23
0.47
-
0.23
0.22
0.29
0.34
0.72
The primary indication of the research was that plants work better in ‘acoustically
live’ spaces, i.e. those that have minimal soft furnishings. In domestic terms it
makes sense for those who favour the minimalist approach to room decoration,
and who, quite possibly, have hard flooring, to use a range of (larger) heavily
foliaged plants to make up for deficiencies in room reverberation (RT60). Yet,
whilst this may be true, anecdotal evidence suggests that more normally
furnished rooms can still benefit from the absorbent and diffuse action of plants.
The proviso being that there be some room around the plant(s), rather than they
being used right next to an absorbent piece of furniture, for example, where any
acoustical action they may have will be lost.
In terms of use and placement. It isn’t recommended that a plant (plants) be used
to treat first reflection (mirror) points – unless, through domestic consideration,
there is no alternative. Where plants tend to come into their own is in the
treatment of room shape issues and secondary reflection (mirror) points. One
such location is the bay window. An often unused area of floor space – yet one of
audible sonic detriment – it makes the ideal location for a (perhaps tablemounted) plant or two. By combining a stiffer, architectural, vine with something
of a lighter, but dense foliage – such as the Weeping Fig. You will generate a
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(relatively) wide bandwidth of absorption, as well as varying degrees of
reflection/diffraction/diffusion, that may ameliorate known bay window issues.
Rooms corners are an ideal location for the use of larger plants. Fortunately,
corners also maximise the potential benefits plants have to offer. Anecdotal
evidence suggests that a larger palm in each of the room corners to the rear of
the seating position can be as effective as OEM shaped foam in terms of modal
treatment. Smaller plants, on the other hand, are ideal for spot treatment. A
mantelpiece will hold a number of plants that along with their round-profile pots,
add modest diffusion to a chimney brest. Likewise, placed on shelves on the front
wall (alcoves), behind, and to the sides of the loudspeakers.
Beyond this, experiment. In the knowledge that as good as they are for general
ambience, plants have acoustical benefit, too. They may not be as effective as
OEM treatment, but they do have effect, and one that is a viable, and useful
domestic alternative.
Bibliography and links.
Master Handbook of Acoustics – F. Alton Everest.
Sound on Sound (magazine) – Various.
http://www.rpg-europe.co.uk/
(Manufacturer and retailer of OEM acoustic products)
http://www.decorative-art-services.co.uk/
(Fine art acoustic panels)
http://www.mcsquared.com/problems.htm
(A useful resource for downloadable RT60 comparison samples)
http://www.plants-in-buildings.com/
(Includes research into the acoustical properties of plants)
http://www.behringer.com/
(Audio electronics manufacturer)
And finally…
A note of thanks must go to Alex Schouvaloff of Decorative Art Services, and to
Mathew Moule of RPG Europe.
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